As is known, the principle of recirculating the burnt gases (EGR) lies in introducing a given amount of exhaust gases in place of some of the fresh air on the inlet side of diesel or gasoline engines. External EGR systems use a valve to do that, which valve taps off some of the exhaust gases and reintroduces them into the inlet side. This principle makes it possible to limit the emissions of NOx (the greater the fraction of gas recirculated, the greater the reduction in emissions) but reduces engine performance.
It is necessary to draw distinctions between two methods by which this solution is employed: high-pressure recirculation and low-pressure recirculation. While low-pressure recirculation consists in tapping exhaust gases off after the turbine and reintroducing them before the charge air compressor (the gases sometimes also being reintroduced upstream of the intercooler that follows the compressor), high-pressure recirculation (turbocharged engines) on the other hand consists in tapping exhaust gases off upstream of the turbine and reintroducing them downstream of the compressor. High-pressure recirculation therefore has the advantage, over low-pressure recirculation, of requiring a shorter circuit (just one valve, rapid system response). The pressure difference between the exhaust and the inlet side is used as the driving force, the flow rate being regulated by the valve.
At the present time, in the case of a high-pressure EGR circuit associated with a diesel engine that has low compression ratios of no higher than 16, it is becoming increasingly difficult to perform a cold start because of the inferior thermodynamic conditions (lower temperature and pressure) in the cylinder. In the text which follows, a cold start is to be understood as being a start performed in a range of temperatures starting from −30° C. There is therefore a need to improve the starting of the most recent-generation turbocharged diesel engines with low compression ratios.
Exhaust gas recirculation capable of adapting to low temperature conditions in low engine speed phases is known from document EP 1 138 928. That document preferably envisions starting the engine with the EGR deactivated, the opening of the EGR valve being initiated when the engine is receiving a sufficient charge. Alternatively, the EGR valves can be left open during starting. However, in such cases, the engine often stalls through lack of oxygen just after it has started. Even if the valves are left open just a small amount in order to reduce the recirculation, the engine cannot then run in a stable manner at idling speed.
Because of the various difficulties encountered, the prior art therefore envisions closing the EGR valve when cold, as per the teachings of document EP 1 101 917 in particular.
Other solutions, for engines with low compression ratios, consist in using a high-speed ceramic glow plug or in enhancing the technical design of the inlet side. These solutions are still not well enough optimized to guarantee that the engine will make a problem-free cold start.